1. Field Of the Invention
The present invention relates generally to video recording and playback. More particularly, the invention relates to data formats for optical writing and reading of data with respect to a storage medium such as a flexible optical tape.
2. State of the Art
Presently, flexible tape for optical storage of data is known, as described in U.S. Pat. Nos. 4,719,615 and 4,912,696, both identified on their faces as being assigned to Optical Data, Inc. To optically record data on a medium such as a flexible optical tape, a beam of laser light is directed onto the tape. The laser beam melts the tape or burns holes in the tape to produce data spots representing bits of data. The data spots have a reflectivity, transmissivity or other optical characteristic which can be distinguished from the background of the optical tape.
Optically sensitive tape has not been effectively used to record high bandwidth signals. Thus, the commercial potential of optical tape to record signals such as television picture signals has not been effectively exploited.
Generally speaking, television pictures are comprised of snapshot-like "frames" that contain video signal information in horizontal scan lines organized by synchronizing signals. For example, according to the standards of the National Television Systems Committee (NTSC), each frame of video information comprises 525 horizontal scan lines. Further according to NTSC standards, the frame repetition rate is thirty frames per second, or 15,750 horizontal scan lines per second.
In practice, all of the video information in a complete television frame is not reproduced at a receiver simultaneously; instead, a technique known as interlaced scanning is used to reduce flicker. In interlaced scanning, each frame of video information is divided into two interlaced fields, each comprising a raster array of odd numbered or even numbered horizontal scan lines. Thus, if the horizontal scan lines of a 525-line NTSC frame were numbered sequentially from the top of a raster array, an odd-line field would comprise numbered horizontal scan lines 1, 3, 5, and so forth through frame line 525. Similarly, an even-line field would include numbered lines 2, 4, and so forth through line 524. According to the NTSC format, the field repetition rate is sixty fields per second.
In video recording systems that employ magnetic videotape as the recording medium, it is conventional to record each television field by using only a single track on the recording medium. This single track is traced by a recording head which helically scans the recording medium. (Helical recording heads are widely used because they provide high head-to-tape speed, usually exceeding one meter per second, with relatively slow moving tape.) FIG. 1 shows an example of four fields of video information recorded in parallel tracks 2, 4, 6 and 8, respectively, which extend at a small angle (e.g., 5 to 15 degrees) relative to the longitudinal edge of a magnetic videotape 10.
However, optical recording and playback of data poses significant problems which do not arise in traditional magnetic recording playback systems. For example, use of a laser to establish data spots representing bits of digital data or analog data on the flexible tape must be selected with practical limitations in mind. More particularly, in establishing relative movement between a laser and a flexible tape, there i.e. a limit to how rapidly the optical characteristics of a spot on the tape can be altered to form a data spot. Such a limit can be significant when, for example, the information being recorded represents a variety of different bandwidth signals; one of which corresponds to high frequency video information. Further, such a limitation can be significant when attempting to optimize space efficiency in recording information on the optical tape.
It would therefore be desirable to provide an optical system capable of writing and reading various bandwidth signals onto flexible optical tape. Further, it would be desirable to provide a cost effective system which could exploit this capability in a commercial environment such as television signal recording and playback for an optical VCR.